This invention relates, in general, to encoders/decoders, and more particularly, to those encoders/decoders employing continuously variable slope delta modulation.
In the communications field it is often desirable to convert audio analog signals to digital signals for transmission purposes. Signal-to-noise ratios do not vary with distance in digital transmissions. In addition, multiplexing, switching, and repeating hardware which is used for digital transmission is more economical and ofter simpler than having individual lines to carry analog signals in addition to analog handling equipment. However, instrumentation analog-to-digital converters do not always meet communications performance requirements. Continuously variable slope delta modulation analog-to-digital converters are well suited to the requirements of digital communications and are an economically efficient means of digitizing analog inputs for digital transmission.
The innermost control loop of a CVSD converter is a simple delta modulator. A simple delta modulator consists of a comparator in the forward path and an integrator in the feedback path of simple control loop. The inputs of the comparator are the input analog signal and the integrator output. The comparator output reflects the sign of the difference between the input analog voltage and the integrator output. That sign bit is a digital output and controls the direction of ramp in the integrator. The comparator is clocked so as to produce a synchronous and band limited digital bit stream. If the clocked serial bit stream is transmitted to a similar integrator at a remote point, the remote integrator output is a copy of the transmitting control loop integrator output. To the extent that the integrator at the transmitting location tracks the input signal, the remote receiver reconstructs the input signal. Low pass filtering at the receiver output will eliminate most of the quantizing noise if the clock rate of the bit stream is an octave or more above the bandwidth of the input signal. Voice bandwidth is typically 4 KHz and clock rates from 8 KHz and up are possible. Thus, the delta modulator digitizes and transmits the analog input to a remote receiver. The serial, unframed nature of the data is ideal for communications networks. With no input at the transmitter it is customary to transmit a continuous one-zero alternation bit stream. By making both the transmitting and receiving integrators leaky, then during any loss of contact the receiver output decays to zero and receive-restart begins without framing when the receiver reacquires.
The fundamental advantages of the delta modulator are its simplicity and serial format of its output. Its limitations are its ability to accurately convert the input within a limited digital bit rate. The analog input must be frequency limited and amplitude limited. The frequency limitations are governed by the Nyquist rate while the amplitude capabilities are set by the gain of the integrator. For a given signal level, one specific gain will achieve an optimum noise level. Unfortunately, the basic delta modulator has a small dynamic range over which the noise level is constant. In the past, the dynamic range of the basic delta modulator was increased slightly by employing a digital algorithm in conjunction with a low pass filter to control the gain of the integrator of the delta modulator. Although such a scheme did produce slight gains in the dynamic range, it was an open-loop control which did not provide a maximum obtainable dynamic range.
Accordingly, it is an object of the present invention to provide an improved encoder/decoder using continuously variable slope delta modulation.
Another object of the present invention is to provide an encoder/decoder having a dynamic range, with flat signal-to-noise ratio, on the order of 50 dB.
Yet another object of the present invention is to control gain of a delta modulator/demodulator by comparing a companding ratio of a continuously variable slope delta modulation to a companding ratio reference.
Another object of the present invention is to provide a closed-loop controlled companding ratio reference which improves the dynamic range of continuously variable slope delta modulation.
A further object of the present invention is to provide a control for continuously variable slope delta modulation circuit having improved temperature stability.